Washington has evidence that Tehran is trying to buy new equipment for a key nuclear facility. But the White House isn't willing to say anything publicly about it.

The United States has privately accused Iran of going on an international shopping spree to acquire components for a heavy-water reactor that American officials have long feared could be used in the production of nuclear weapons-grade plutonium.

A U.S. delegation informed a U.N. Security Council panel of experts monitoring Iranian sanctions in recent months that Iranian procurement agents have been increasing their efforts to illicitly obtain equipment for the IR-40 research reactor at the Arak nuclear complex.

The American allegations, which have never before been reported, come more than a year after the Iranian government pledged as part of an interim agreement with the United States and other big powers to scale back Iran’s most controversial nuclear-related activities, including the enrichment of high-grade uranium, in exchange for billions of dollars in sanctions relief. They stand in stark contrast to recent remarks by Secretary of State John Kerry, who has repeatedly credited Tehran with abiding by the terms of the November 2013 pact, which bound Tehran to suspend some of its work at Arak. “Iran has held up its end of the bargain,” Kerry said last month in Vienna as he announced a seven-month extension of the timetable for big-power talks.

The allegation is also sure to add to the mounting congressional unease over the administration’s ongoing talks with Tehran. Many lawmakers from both parties believe that the White House is making too many concessions to Tehran to cement a deal that it sees as central to the president’s legacy. With the GOP slated to take over the Senate next month, Iran hawks like Arizona Republican John McCain and Illinois Republican Mark Kirk are already promising to push through a new package of economic sanctions, a move that the White House believes would scupper the delicate talks with Tehran. Both men are likely to see the new U.N. allegations as proof that Tehran simply can’t be trusted to abide by the terms of a future deal.

As part of its pact with the United States and other big powers, Iran has halted some critical construction work at the IR-40 research reactor, which is already being monitored by the International Atomic Energy Agency (IAEA), and in December provided the agency’s inspectors with “managed” access to a heavy-water production plant at Arak that had not been subject to IAEA monitoring. Iran is currently in discussions with the IAEA on establishing a protocol for future monitoring of the reactor.

The U.S. allegations were detailed in a confidential Nov. 7 report by an eight-member panel of experts that advises a U.N. Security Council committee that oversees international compliance with U.N. sanctions on Iran. The report, which cites an unnamed state as the source of the allegation, doesn’t identify the United States by name. But diplomatic sources confirmed that the United States presented the briefing.

The confidential report, portions of which were made available to Foreign Policy, notes that “one member state highlighted during consultations with the panel a number of developments regarding proliferation-sensitive procurement by Iran.” The delegation, the report continued, “informed the panel that it had observed no recent downturn in procurement” in recent months. It did cite a “relative decrease in centrifuge enrichment related-procurement” in recent months. But it added that it had detected “an increase in procurement on behalf of the IR-40 Heavy Water Research Reactor at Arak.”

The United States indicated that foreign businesses and purchasing agents interested in doing business with Iran have been taking advantage of the improved diplomatic atmosphere to broker new deals with Iran. At the same time, they say there is overwhelming evidence that Tehran continues to transfer huge amounts of weapons to its proxies and allies, including Syria and Iraq. In June, the U.N. panel of experts asserted that an Iranian shipment of rockets, mortars, and other arms seized in March by the Israeli navy while en route to Sudan violated the U.N. arms embargo. Only last week, U.S. and Iranian officials confirmed that Iranian warplanes had launched airstrikes on Islamic State targets in Iraq, making Tehran and Washington unofficial allies in the fight against the Islamist group.

The U.S. mission to the United Nations declined to comment on the matter. A spokesman at the Iranian mission to the United Nations did not respond to a request for comment.

The American claims have been greeted with skepticism by some outside observers, who point to the history of U.S. intelligence failures during the run-up to the Iraq War. Kelsey Davenport, the director for nonproliferation policy at the Arms Control Association, a Washington-based group that favors a nuclear deal with Iran, said there is cause to be cautious about Washington’s latest allegations.

In the past, evidence of Iranian smuggling has often come to light months or years after the crime has occurred, which means that the sanctions violations cited by the Americans may “predate the interim agreement,” she said. Davenport also pointed out that much of Iran’s illicit procurement activities are conducted by private entrepreneurs and regime hard-liners who, like their conservative counterparts in Washington, may be keen to scuttle a historic nuclear deal.

She said that Tehran had scrupulously upheld its commitment to the United States and its diplomatic partners to suspend activities at Arak and fashion an agreement with the IAEA to ensure that the site is never used to develop nuclear weapons. Davenport added that a Feb. 20 report by the IAEA found, in her words, that “there is no activity ongoing at the reactor itself.”

Iran’s alleged quest to acquire parts and other equipment for Arak is part of a long-standing campaign by Tehran to evade international sanctions and to pursue what it claims is a peaceful nuclear program. Over the past decade, Iran has responded to U.N. sanctions on its nuclear program by developing one of the world’s most confounding shell games, using front companies, international middlemen, and clandestine shippers to secretly acquire the industrial raw materials — aluminum rods, vacuum pumps, and carbon fibers — needed to keep its nuclear reactors running smoothly.

But as the world’s big powers prepare for a final round of talks on the fate of Iran’s nuclear program, many foreign governments from Asia to the Middle East have effectively stopped reporting Iranian violations to the United Nations, according to several U.N.-based diplomats.

In its November report, the panel, which is charged with tracking Iran’s nuclear purchasing program, said that they had not received a single report from a U.N. government of a sanctions violation by Iran between July and November 2014. In the past, the U.N. has investigated dozens of cases of sanctions violations involving businesses in countries ranging from Nigeria to China.

The report’s authors suggested that countries may have been reluctant to report Iranian violations of the sanctions in deference to Iran’s new leader, Hassan Rouhani, who was elected president in June 2013 on a pledge to repair relations with the world and secure an end to years of crippling economic sanctions, and out of concern that doing so could risk upsetting sensitive nuclear talks.

Back in June, the panel of experts noted that several governments informed it that they had observed a “decrease in the number of detected attempts by the Islamic Republic of Iran to procure items for prohibited programs, and related seizures, since mid-2013.” The panel suggested that “it is possible that this decrease reflects the new political environment in the Islamic Republic of Iran and diplomatic progress towards a comprehensive solution.”

A Security Council diplomat familiar with the panel’s findings said the world body was “aware of the fact that there had been no reported violations during the last [reporting] period and that was a matter of concern.

“We did think it was likely that people had been holding back on reporting because of concerns about the impact it might have on the talks,” the diplomat said.

“There has been a drop-off of reported cases to the [U.N.] Security Council,” added Ian J. Stewart, who heads up the Project Alpha at King’s College London, which tracks Iranian proliferation. “We follow this very closely, and I’m not aware of any specific cases in the last 12 months. As I understand it, the panel of experts doesn’t have much to do. It raises the question: What’s happening?”

There are various conflicting theories making the rounds in U.N. circles. Iran may have developed a more sophisticated system for concealing the illicit trade, or it may have simply slowed down its illicit activity to avoid confrontations with the West that could undercut prospects for a nuclear deal under negotiation with the world’s six key powers that could ease its economic isolation and reset its relationship with the United States. The temporary diplomatic pact in place since last November — which included a commitment by Tehran to suspend some of its more advanced enrichment activities — may have simply reduced Tehran’s need for foreign supplies for parts of its nuclear program, particularly for the enrichment of uranium.

The current slowdown in activity by the panel contrasts with the flurry of international reports describing the ongoing seizure of sensitive materials at sea. In February, British authorities arrested a Chinese national, Sihai Cheng, on U.S. charges of shipping huge batches of so-called “pressure transducers” to Iranian clients through a Shanghai subsidiary of a Massachusetts firm, MKS Instruments, Inc. On Friday, he arrived in Boston, where he will face charges of supplying Iran’s nuclear program. A lawyer for MKS, Kathleen Burke, said in an email that the Chinese businessman “is not, and has never been, an employee of MKS or any of our subsidiaries.” She added that MKS itself “is not a target of the investigation and has been providing assistance to the government throughout the investigation.”

The devices — which are prized for the ability to withstand exposure to corrosive uranium hexafluoride gas — can be used to regulate gas pressure in a centrifuge as uranium is being enriched. As an experiment designed to show how easy it is to obtain the banned device, Stewart, of King’s College, purchased one from a Chinese distributor on eBay.

The apparent decline in sanctions-evading activities has not translated into increased cooperation with the U.N. or the IAEA. Tehran has largely ignored edicts emerging from the U.N. Security Council, which it derides as an illegitimate tool of the United States and its European allies. While Iran has authorized increasing numbers of inspections by experts at the IAEA, meanwhile, it has yet to comply with the U.N. nuclear energy agency’s requests for information about the Iranian military’s involvement in the country’s nuclear program.

Tehran has benefited from diplomatic assistance from Moscow, which routinely blocks action by the council designed to put pressure on Iran to comply. Russian diplomats argue that any steps to confront Iran may threaten to undercut sensitive nuclear talks. Over the long haul, Russia believes that the U.N. Security Council should leave it up to the IAEA to monitor Iran’s compliance with any new arrangement aimed at ensuring Tehran doesn’t pursue a nuclear weapons program.

The U.N. Security Council first imposed U.N. sanctions on Iran in 2006 in an effort to compel Tehran to halt the development of its nuclear program, including through a freeze on its enrichment of uranium, until it provided verifiable assurances that it was not pursuing a nuclear weapons program.

The measures, which were expanded over subsequent years, also banned the development of ballistic missiles capable of delivering a nuclear payload, prohibited the transfer of most conventional weapons, and imposed travel bans and an asset freeze on individuals linked to Iran’s banned programs. Under the leadership of the Obama administration, the Security Council in 2009 established a panel of experts to enforce the sanctions.

While Iran has long denied it is pursuing a nuclear weapon, it has publicly boasted about its ability to circumvent U.N. and Western sanctions aimed at denying its ability to enrich uranium. “Of course we bypass sanctions,” said Hassan Rouhani, Iran’s president, during a televised address to the nation last August. “We are proud that we bypass sanctions, because the sanctions are illegal.”

Iran’s defiance reflects a deep antipathy towards the U.N. Security Council, which it claims lacks legitimacy. Tehran has argued that the 15-nation council overstepped its authority by enforcing limits on its nuclear program.

Indeed, Iran has been trying to get the United Nations off its back. During the past decade, Iran has been targeted by successive rounds of U.N. sanctions aimed at suspending its nuclear program, halting its trade in weapons, and foiling its efforts to acquire supplies that could be used for a possible nuclear weapon. Movement to alleviate U.N. sanctions is particularly important for Iran as they have less confidence that the U.S. leadership has the power to overcome Congressional push for sanctions. U.N. sanctions have proven particularly grating because they challenge Iran’s right to enrichment uranium, and provide a political and legal rationale for all states to constrain Iran’s commercial and military activities. Iran has stepped up efforts to obtain relief from sanctions. Russia has grown increasingly vigorous in blocking action in the sanctions committee. It has also indicated that the panel of experts should be shuttered as part of a final deal.

“The Iranians have recently come to this notion that they have to get the U.N. Security Council sanctions removed immediately,” said Robert Einhorn, a former top State Department advisor on nuclear proliferation issues during the Obama administration who is currently a senior fellow at the Brookings Institution.

Einhorn said U.S. policymakers expected Iran to work hardest to eliminate the painful American and European sanctions targeting Tehran’s lucrative oil, banking, and shipping sectors. “The United States had the assumption that the Iranian would see relief from those sanctions as the highest priority and the U.S. prepared to suspend some of them using executive powers immediately,” he said. But he said the getting relief from the U.N. sanctions, not the American ones, had recently emerged as the “top” Iranian priority.

The risk, Einhorn said, is that an abrupt lifting of U.N. sanctions could offer Iran a path to acquiring equipment for a nuclear weapon. “The U.S. and its partners know Iran procures this stuff illicitly and they don’t want the restrictions on that removed prematurely,” he said.

Nuclear proliferation experts have pressed the United States and other big powers to ensure that any lessening of sanctions doesn’t weaken the West’s ability to prevent the diversions of materials to a covert nuclear weapons program.

David Albright, the executive director of the Institute for Science and International Security, and Olli Heinonen, a senior fellow at Harvard University’s Belfer Center for Science and International Affairs and the former head of nuclear safeguards for the IAEA, proposed that the U.N. Security Council continue to sanction dual-use goods that could be used for a nuclear weapons program while establishing an internationally monitored channel for the sale of legitimate equipment needed for a civilian nuclear energy program.

The plan, they suggested, would need to include safeguards designed to insure that the dual-use equipment cannot be diverted to a covert nuclear weapons program. “Currently, the world is on heightened alert about Iran’s illicit procurements for its sanctioned nuclear, missile and military programs,” they wrote. “But as nations enter into expanded commercial and trade relationships with Iran, a risk is that many countries will effectively stand down from this heightened state of awareness and lose much of their motivation to stop banned sales to Iran even if U.N. sanctions remain in place.”

But the prospects of continued U.N. sanctions are going to be hard for Iran to withstand. “It is very difficult for the Iranians to accept that they are not going to get any of the big sanctions lifted at an early stage because it would leave them in a vulnerable position as they defend the deal back in Iran,” said Trita Parsi, an expert on Iran’s nuclear program and occasional contributor to Foreign Policy. “The way the Iranians see this is they are being asked to make immediate, painful concessions but the relief is going to come quite late on sanctions. [President Hassan] Rouhani needs to be able to show tangible economic impact relatively soon after the deal because there are going to be elements in Iran that are going to go after the deal.”

Parsi said that American negotiators went into the talks signaling that they planned to provide limited relief from U.N. sanctions. It remains unclear how much Washington has yielded on the topic in recent weeks. “From the Iranian perspective the view is that the [U.S.] president will have greater difficulty lifting U.S. sanctions as result of a Republican Senate,” he said. Tehran believes that “something has to be done on the U.N. or the European side, and their preference is for the U.N. side. At the end of the day something has got to give.”